A prolonged period of asystole is an unambiguous indication for pacing. The more complex issue is what pacing rate is appropriate for the heart and when and under what circumstances should the pacing therapy stop. The traditional demand pacemaker sensed R-wave intervals on a beat-to-beat basis and paced only when the underlying rhythm was below a so called escape interval. Modern therapies for bradycardia are much more complex. Many modern pacemakers have sensors. These sensors provide more information about the cardiovascular state of the patient and efforts are being made to use this additional information to improve the efficacy of pacing therapy. These improvements also expand the indications for pacing. At present, pacing therapies have been proposed for treating vasovagal syncope.
Vasovagal syncope, also called neurally-mediated or neurocardiogenic syncope, is a relatively common entity. It also goes by the name "common faint." For most individuals who are subject to this entity, it occurs very infrequently and can be managed by sitting or lying down when warning symptoms, such as lightheadedness, sweating and nausea, occur. When individuals experience repeated episodes without the usual warning signs, then pharmacologic or pacing therapy is required.
The common "faint" is an example of an interaction between the automatic nervous system and the cardiovascular system. There are several types of faints of which vasovagal syncope is one. Within vasovagal syncope, there are subsets which differ in detail. The typical episode involves a concurrent and precipitous drop in both blood pressure and heart rate. For an ambulatory subject, the resultant sudden loss of cardiac output can result in a potentially injurious fall.
Drugs have been considered the first line of therapy for many of these patients. However the chronic use of drugs for rare episodes of vasovagal syncope is problematic. More recently pacing therapies have been proposed for these patients. For example DDI pacing with hysteresis has been explored as a therapy for this patient group. With DDI hysteresis pacing, a patient with normal sinus function can remain in sinus rhythm most of the time. Vasovagal events which trigger the hysteresis escape interval result in pacing at a relatively high rate to compensate for both asystole and vasodilatation.
It is also possible to trigger a pacing therapy based upon a disease specific rate drop algorithm. See for example U.S. Pat. No. 5,501,701 to Shelton et al. and U.S. Pat. No. 5,501,701 to Markowitz et al. As taught by the patents, a drop in heart rate over a relatively short interval invokes pacing at a programmed higher pacing rate. However, the conventional rate drop algorithm requires several beats before the pacemaker intervenes which is undesirable for a presyncopal patient. Both conventional DDI with hysterisis pacing and the rate drop algorithm are unable to reliably distinguish pathologic from physiologic rate drops. Therefore, the prior pacemakers may pace inappropriately under some circumstances which is undesirable.